1. Field of the Invention
This invention relates to a system which employs vortex leveraging tabs to thereby reduce the hazard posed by the vortex wake of large aircraft to other aircraft flying nearby.
2. Description of Related Art
The wings of airplanes generate strong, concentrated vortices that trail from the area near their tips. These vortices generally arise from air spilling over the edges of each wingtip due to the difference in pressure on the upper and lower surfaces of the wing. In some cases, the wingtip vortices are fully formed (or "rolled up") immediately downstream of the trailing edge of the wing, while in other cases they form more gradually over a distance of one or more wingspans. The direction of rotation of these vortices when viewed from behind (downstream of) the wing are opposite one another, and the resulting "vortex pair" constitutes the most important feature of the vortex wake of the aircraft. In the absence of significant outside disturbances such as strong atmospheric turbulence, these vortex pairs often persist for several minutes, and this can pose a safety hazard to other aircraft. For example, a small airplane following close behind a larger one that generates a strong vortex pair (e.g, a commuter plane or small jet following a large jetliner) can in some cases be flipped over by the strong swirling flow of this pair. This vortex wake hazard is particularly serious for aircraft flying at low speeds and close together, and thus it is one of the principal constraints on the frequency with which jet transports can land at major airports.
The vortex pairs generated by airplane wings are in fact inherently unstable and eventually break up due to the growth of disturbances introduced directly or indirectly by atmospheric turbulence. The pair often breaks up into a series of coarse vortex rings and these rings are, in turn, susceptible to a further set of instabilities that continue in a cascade that ends with the vortex wake dissipating into the background flow over a period of several minutes. While the eventual wake breakup is effectively guaranteed, it is often unacceptably slow from the point of view of enabling commercial jet transports to fly safely close together when on approach to airports, and thus they often cannot land with sufficient frequency to avoid air traffic delays. Hence, the development of methods to accelerate the breakup of the vortex pairs trailed by aircraft wings is of considerable practical importance.
The prior art includes both studies in the technical literature that provide basic descriptions of vortex wake behavior (See, for example, S. C. Crow, "Stability Theory for a Pair of Trailing Vortices," AIAA Paper No. 70-53, January 1970 and V. R. Nikolic and E. J. Jumper, "Attenuation of Airplane Wake Vortices by Excitation of Far-Field Instability," AIAA Paper No. 93-3511, AIAA Applied Aerodynamics Conference, pp. 876-884, 1993) as well as several aircraft operational concepts directed at addressing the problem of wake hazard by weakening or diffusing the individual wingtip vortices (e.g., S. C. Crow, Panel Discussion in Aircraft Wake Turbulence and Its Detection, J. H. Olsen, ed., Plenum Press, New York, p.377 ff., 1971: D. Croom and R. E. Dunham, "Low Speed Wind Tunnel Investigation of Span Load Alteration, Forward Located Spoilers, and Splines as Trailing Vortex Hazard Alleviation Devices," NASA TN D-8133, 1975; and H. F. Faery, and J. F. Marchman, "Effect of Whitcomb Winglets and Other Wingtip Modifications on Wake Vortices," Proc. of the Aircraft Wake Vortices Conference, J. N. Hallock, ed., Report No. FAA-RD-77-68, pp. 207 -216, June 1977). In assessing this literature, it is important to realize that the hazard posed to other aircraft by the vortex wake is best measured by the torque or moment that is imposed on following aircraft by the persistence of organized vortices with strongly swirling flow. The torque or moment exerted by the vortices is an integrated property whose influence is summed up from contributions from velocities induced along the full span of the wing of the following aircraft. An effective wake mitigation method or device must break up the organized vortical flow in the vortex, reducing it to eddies whose scale is small compared to typical aircraft wings. It is not adequate to simply reduce the peak velocity inside the vortex (See for example, A. J. Bilanin and S. E. Widnall, "Aircraft Wake Dissipation by Sinusoidal Instability and Vortex Breakdown," AIAA Paper No. 73-107, January 1973; V. J. Rossow, "On the Wake Hazard Alleviation Associated with Roll Oscillations of Wake-Generating Aircraft," AIAA Paper No. 85-1774, AIAA 12th Atmospheric Flight Mechanics Conference, pp. 78-88, 1985; and V. J. Rossow, "Prospects for Destructive Self-Induced Interactions in a Vortex Pair," Journal of Aircraft, Vol.24, No.7, pp.433-440, July 1987).
The prior art also includes several inventions that have attempted to deal with the problem of vortex wake mitigation. U.S. Pat. No. 3,845,918 to Richard P. White, Jr. describes an additional surface mounted on the tips of a wing or hydrofoil that is aligned with the free stream and occupies 0.3 to 0.6 of the tip chord, from roughly mid-chord of the tip to the trailing edge. The purpose of this fixed surface--in combination with the effect of the rolling-up tip vortex--is to produce an alteration in the flow near the tip of the wing that yields a stall angle of attack relative to the additional surface and that dissipates the strength of the vortex. This is one example of a device that decreases the peak level of swirling velocity in the vortex immediately downstream of the wingtip, but which does not significantly reduce the overall strength of the vortex.
U.S. Pat. No. 4,477,042 to Roger W. Griswold, II describes contouring of the wingtip shape to thereby smooth the merging of flow between the upper and lower surfaces of the wing. The primary aim of this invention is alleviate the wake of a lifting wing, though a secondary goal is improving wing efficiency by decreasing the drag generated by the wing. An additional feature of this invention is a related concept involving the closure of gaps between partial span flaps. This also smoothes the flow around the edges of flap segments, weakening the vortices trailed from exposed edges of the flaps in a manner similar to the effect produced on the vortices trailed from wingtips. U.K. Patent 2,051,706 issued to British Aerospace describes a similar invention where an array of vane-type devices are proposed for the diminution of vortices generated by segmented flaps.
U.S. Pat. No. 4,190,219 to James E. Hackett details a vertical lifting surface, swept slightly aft, mounted downstream of the trailing edge of the tip of a lifting wing. The stated intent of this and related inventions is to preclude the formation of a distinct trailing vortex, shedding instead weaker discrete vortices that produce less kinetic energy in the wake and induce less drag on the generating wing by virtue of the reduced swirling velocity. Hackett relies on a single tip-mounted vane to accomplish this, while U.S. Pat. No. 4,017,041 to Wilbur C. Nelson describes an invention consisting of multiple retractable foils with the same object.
U.S. Pat. No. 4,046,336 to James L. Tangler describes the use of a fixed sub-wing attached to the tip of a lifting wing or rotor blade. This sub-wing is designed to divide the vorticity generated at the tip of the lifting surface so that two vortices are formed with a separation of 25% to 50% of the chord of the wing or blade. The interaction of the vortex generated by the sub-wing and that originating from the trailing edge of the wing or blade itself has the effect of diffusing the resultant vortex that forms from the amalgamation of these two, yielding lower swirl velocities.
U.S. Pat. No. 5,492,289 to Daniel M. Nosenschuck et al. describes a general method for shaping a lifting body to produce reduced strength trailing vortices. The preferred embodiment is a lifting wing with perturbations imposed on its trailing edge to produce the desired reduction in vortex strength. U.S. Pat. No. 4,697,769 to Ron F. Blackwelder et al. describes a method for achieving a similar result for the particular case where strong vorticity is generated at the leading edge of wings from the presence of flow separation (for example, delta wings at high angle of attack). This invention identified several different ways to use unsteady, periodic disturbances to increase or decrease the lift on such wings, thus implying a way to change the strength of vortices trailed into the wake. However, wings that generate vortices at the leading edge are a small subset of all aircraft, and, hence, this is not a method that is generally applicable to conventional aircraft configurations.
Other examples of prior art include the use of active flow control to induce hydrodynamic instabilities in individual vortices. U.S. Pat. No. 3,881,669 issued to Martin Lessen describes the use of devices that inject high velocity air into the central "core" region of a vortex in a way that instigates breakup of the core structure of individual vortices. This method involves introducing a flow of air through a small nozzle at or near the wingtip into the core of a trailing vortex in a direction that is collinear and coaxial with the vortex, and with a momentum flux of sufficient magnitude to render the vortex hydrodynamically unstable. Flow control devices of this kind are generally difficult to implement, requiring significant additional mechanisms to add the desired mass flow and being sensitive to the position of the introduction of the flow.
The prior art in this area thus consists primarily of techniques for weakening the tip vortices, often by amounts that produce significant performance improvements in terms of the drag induced on a wing by the wake but which do not significantly mitigate wake vortex hazard. Again, the crucial circumstance that drives this result is that devices designed primarily to weaken or diffuse a vortex chiefly only diminish the swirl velocity near the central axis or "core" of the vortices, doing little to reduce the overall circulation strength of the vortex. Though such devices can diminish the peak velocities encountered downstream, the net rolling moment experienced by a following aircraft is much less strongly affected since this rolling moment is an integrated quantity distributed over the full span of the following aircraft. This result is evident in the indifferent success of devices in flight tests described in the technical literature, many of which produce well-diffused vortices but which do little to reduce the total moment on trailing aircraft. None of these inventions have proved to be so effective as to be put into practice.
A much more appropriate and attractive method for reducing vortex hazard is the acceleration of the instabilities in the wake arising from the mutual interaction of vortices in the wake. Such interactions are in general the actual mechanism that dissipates the wake when appropriately excited by atmospheric turbulence, and a logical method for accelerating this process is to employ an active mechanical system for exciting the most unstable modes of motion in the wake. Prior technical publications have described methods for exciting the Crow instability of the single vortex pair (e.g., both S. C. Crow articles, Supra). These include introducing time-varying control inputs to the aircraft to cause it to bob up and down or roll laterally at frequencies characteristic of Crow instability (typically, the time for the aircraft to traverse approximately eight wingspans of distance) (ref. A. J. Bilanin and S. E. Widnall article, Supra). This approach can be effective in instigating the Crow instability, but is very uncomfortable for passengers and requires large (of the order of 10 percent) changes in the mean lift of the aircraft, which can produce very large stresses on the wing roots. An appropriate solution for the wake mitigation problem would involve a method that does not require or cause large variations in mean loading on the existing aerodynamic surfaces.